I would sort of like to know this, too. I've been "training" Waze two different routes that I take to work. One is faster, according to Waze, by time, but is subject to getting backed up. Multiple times there have been traffic jams where Waze decides it wants to plow through it instead of taking the clear route.
Waze is a wonderful app, but your safety and the safety of the drivers around you comes first!